Agronomic performance of sweetcorn populations derived from crosses between sweetcorn and field corn

The adaptation of sweetcorn (Zea mays L.) to Spanish growing conditions needs to be improved, something that could be achieved by incorporating traits from field corn populations. The aim of the present study was to analyse the performance of sweetcorn populations carrying the genes sugary1 (su1) or shrunken2 (sh2), as well as sugary enhancer double mutants (su1 se1), produced by crosses between sweetcorn donor populations and the Spanish f ield corn populations Lazcano, Oroso and Rastrojero. The three field corn populations plus the su1, su1 se1 and sh2 populations derived from them, as well as the su1, su1 se1, and sh2 donor populations used in the crosses with the f ield corn populations, were evaluated for their agronomic performance. Several hybrids were used as controls. Assessments were made in Zaragoza (NE Spain) and Pontevedra (NW Spain) over two years. The ability of the field corn populations to improve the sweetcorn varieties was not greatly affected by the sweetcorn mutant involved. The field corn populations chosen as donors for improving sweetcorn must depend on the growing conditions in mind. Sweetcorn populations derived Oroso would appear to be the most favourable for northern Spain, while those derived from Rastrojero would appear to be the most suitable for inland Spain, although other field corn populations are still to be examined. Additional key words: maize, shrunken, sugary, sugary enhancer.


Introduction
In some areas of Spain where field corn is not profitable, sweetcorn could be an interesting alternative.However, sweetcorn lacks the necessary adaptations to be able to grow well in some regions of Spain, including the Atlantic coast (Ordás et al., 1994).
The northern coast of Spain is characterized by a mild, rainy Atlantic climate with a summer drought, while inland Spain has cold winters, hot summers and almost no rainfall during the maize growing period (making irrigation for summer crops a necessity).The growing conditions in these regions are therefore very different.The adaptation of sweetcorn to the northern Spanish coast needs to be improved since wet, cool spring seasons are associated with poor seedling vigour, small ear size, low yield and non-uniform maturity (Ordás et al., 1994).For the specific conditions of northern Spain, Cartea et al. (1996a) proposed the use of the field corn population Oroso for improving the yield and yield components of su1 sweetcorn hybrids.Oroso is also the best potential donor of favourable alleles for sweetcorn survival under cold conditions (Revilla et al., 1998).Another promising field corn population for improving the adaptation of sweetcorn to northern Spain is Lazcano; out of 12 flint corn populations it achieved the best yields across different locations in Spain and France (Malvar et al., 2005).
Lazcano performs well on both the northern Spanish coast and in the interior (Malvar et al., 2005).However, Rastrojero is well adapted to the conditions of Spain's interior and could be a promising source of adaptation traits for sweetcorn in this region; it could also help to improve sweetcorn resistance to corn borers (Malvar et al., 1993(Malvar et al., , 2004)).Resistance to corn borers is required in both regions since ear damage by the corn borer, the European corn borer (Ostrinia nubilalis Hubner) and the pink stem borer (Sesamia nonagrioides Lef.), can greatly reduce the value of the ears (Cordero et al., 1998;Velasco et al., 1999).
Field corn has been used extensively to improve sweetcorn, and successful sweetcorn populations such as Stowell's Evergreen from Ithaca (New York State, USA), Golden Bantam from New York State (USA), and Spanish Gold from Connecticut (USA), have been developed in temperate areas using this strategy (Tracy, 2001).Many studies have shown the potential of flint and dent temperate corn germplasm for improving sweetcorn resistance to European corn borer (Joyce and Davis, 1995), resistance to corn earworm (Guo et al., 2001(Guo et al., , 2004)), stalk and root quality (Treat and Tracy, 1993), agronomic value, adaptation to cold areas with short growing seasons (Cartea et al., 1996a,b;Malvar et al., 1997;Revilla et al., 1998), yield plus agronomic and quality traits (Tracy, 1990), and for breeding new sweetcorn patterns (Davis et al., 1988;Revilla et al., 2000;Velasco et al., 2002).
The genes commonly used to help sweetcorn crops adapt to temperate areas include sugary1 (su1) and shrunken2 (sh2), as well as the sugary enhancer double mutation su1 se1 (Tracy, 2001).Traditional sweetcorn varieties are homozygous for su1, and are characterized by a smooth texture, a creamy endosperm, and the rapid decline of grain table quality after harvest.su1 se1 genotypes produce a high quality, sweet, creamy endosperm, but which is not adequate for post-harvest storage.sh2 genotypes provide kernels with high sugar levels (supersweet) and an extended shelf life, but are associated with poor germination and f ield emergence properties.The aim of the present study was to check the performance of new sweetcorn populations derived from sweetcorn donors carrying su1, sh2 and su1 se1 and three Spanish field corn populations.

Material and Methods
su1, sh2 and su1 se1 sweetcorn donor populations were crossed with the field corn populations Lazcano, Oroso, and Rastrojero (Table 1).The product of these crosses were then self-crossed and homozygous su1, sh2 and su1 se1 seeds selected, thus obtaining new, homozygous su1, sh2 and su1 se1 populations.The original sweetcorn donors were chosen based on their having a flowering period similar to that of the field corn populations used.The field corn population Oroso was crossed with su1, su1 se1 and sh2 sweetcorn donor populations at the Mision Biológica de Galicia-CSIC Centre in Pontevedra.The field corn populations Lazcano and Rastrojero were crossed with su1, su1 se1 and sh2 sweetcorn donor populations at NEIKER (Vitoria) and the Estación Experimental de Aula Dei (Zaragoza) respectively.The three field corn populations plus the new, homozygous su1, su1 se1, and sh2 populations derived from them, as well as the su1, su1 se1 and sh2 sweetcorn donor populations, were assessed for agronomic performance against a number of hybrid controls (Table 1).
Assessments were made in 2003 and 2004 at Pontevedra in northwestern Spain (42° 24'N, 8° 38'W; altitude 220 m; average rainfall 1200 mm), and at Zaragoza (l41°44'N, 0°47'W; altitude 220 m; average rainfall 300 mm) in the Spanish interior.The site at Pontevedra has a sandy loam soil, while the soil at Zaragoza is a Xerosol.Fertilizer, herbicide and insecticide were applied according to practices that would facilitate optimum growth at each location.All plants were flood irrigated over the growing seasons: eight irrigations of about 50 mm each at Zaragoza, and one irrigation at Pontevedra, supplementing the low natural rainfall (average 180 mm during the growing seasons in both years at Zaragoza, and 520 mm at Pontevedra).All cultivation operations, fertilization and pest and weed control were carried out according to local practice.Treatments were arranged in a split-plot design with three replicates.The different population types (i.e., those with wild, su1, su1 se1 and sh2 genotypes) were assigned to different two-row plots [distance between rows 0.80 m, 0.21 m between plants (originally planted as two seeds; one plant later removed as necessary to leave one plant per sowing hole)].The plots were thinned to 60,000 plants ha -1 .Harvesting was manual at both locations.
The traits recorded in 10 plants per replicate were: days to shedding pollen (from sowing to 50% of plants shedding pollen), days to silking (from sowing to 50% of plants silking), plant height (from the ground to the highest node), ear height (from the ground to ear node), stem lodging (percentage of broken stems below the ear), root lodging (percentage of plants leaning more than 45°from the vertical) (Ruiz de Galarreta and Alvarez, 2008), kernel moisture at harvest (g H 2 O kg -1 ) as a percentage of the kernel moisture content (measured using a digital moisture meter), yield (Mg ha -1 at 140 g kg -1 of H 2 O), ear length (cm), ear rows (number of kernel rows per ear), ear health (rated on a 9-point scale taking into account the incidence of fungus and pests; 1 = high incidence, 9 = no damage), and ear shape (percentage of ears with adequate shape).
Analyses of variance (ANOVA) was performed involving the three field corn populations (Lazcano, Composite of hybrids Marvel and 710A (selection in early sowing) EPS18 Composite of hybrids Marvel and 710A (selection in late sowing) Marvel Commercial hybrid Oroso, and Rastrojero) and the new su1, su1 se1 and sh2 populations derived from them.Each year-location combination was considered a different environment.
Environments and replicates were considered random factors, and genotypes (wild, su1, su1 se1 and sh2) and population types (defined by the field corn population present in their pedigrees, i.e., Oroso, Lazcano or Rastrojero) as fixed factors.ANOVA was then performed on the results for each population.Environments and replicates were considered random factors and genotypes fixed factors.Means were compared using the Fisher's protected least significant difference (LSD) method.All analyses were made using the PROC GLM program in the SAS v. 9.1 software package (SAS, 2000).

Results
ANOVA including the three field corn populations, and the new su1, su1 se1, and sh2 sweetcorn populations derived from them, showed significant differences in terms of days to shedding pollen, plant and ear heights, root lodging, yield, kernel moisture, ear rows, and ear health (data not shown).The field corn populations showed a significantly lower kernel moisture content, a greater ear height, greater yield, and healthier ears than any of the new sweetcorn populations (Table 2).In addition, the new sh2 populations produced significantly lower yields, their member plants were shorter, and they showed poorer ear health than any other corn type.The new su1 and su1 se1 populations generally showed intermediate characteristics.
Significant differences were seen among the population types in terms of days to shedding pollen and silking, stem and root lodging, kernel moisture, ear length, and ear rows (Table 3).
Significant differences were seen among su1 populations in terms of days to shedding pollen and silking, plant and ear heights, stem lodging, kernel moisture, ear length, ear rows, and ear shape (Table 4).Significant differences among su1 se1 populations were seen in terms of days to shedding pollen, and silking, plant and ear heights, root lodging, ear length, and ear rows.Finally, significant differences were seen among sh2 Within each column, means followed by the same letter do not differ significantly (P > 0.05). 1 Rated on a 9-point scale taking into account the incidence of fungi and pests (1 = high incidence, 9 = no damage). 20 when ear shape is inadequate and 1 when ear shape is adequate.Within each column, means followed by the same letter do not differ significantly (P > 0.05). 1 Rated on a 9-point scale taking into account the incidence of fungi and pests (1 = high incidence, 9 = no damage). 20 when ear shape is inadequate and 1 when ear shape is adequate.
genotypes in terms of days to shedding pollen and silking, plant and ear heights, stem lodging, and ear rows (Table 4).
The lack of differences among the different populations with respect to yield could be the consequence, at least in part, of the significant effect of the interaction environment × genotype.ANOVA performed separately for each location showed significant differences among genotypes in terms of yield.Among the new su1, su1 se1, and sh2 populations, those derived from Oroso and Rastrojero had the largest yields in Pontevedra and Zaragoza respectively (Table 5).In Pontevedra, Oroso su1 se1 and su1 provided significantly larger yield than the hybrid controls, while Oroso sh2 and the Marvel hybrid produced equal yields.In Zaragoza, the new sweetcorn populations derived from Rastrojero surpassed those derived from Oroso and Lazcano in terms of yield (Table 5), but in general performed less well than the hybrid controls (which are reasonably well adapted to the conditions of the Spanish interior).

Discussion
Backcrossing sweetcorn populations with their corresponding, original field corn populations may improve their adaptation to different regional conditions, although grain quality/yield might be compromised (Tracy, 2001).Further, unfavourable characteristics such as the development of small plants, low yields or high kernel moisture might arise through such genetic modification.In the future, efforts should be made to improve characteristics such as ear length and ear rows, which are currently far from commercial standards.Rastrojero and the new sweetcorn populations derived from it flowered signif icantly later and had fewer ear rows than those derived from Lazcano and Oroso (Table 3).Rastrojero may therefore seem less suitable than the other two field corn populations as a donor of genes for adaptation to the conditions of northern Spain.Earliness is a particular requirement of this area where the growing period is very short.Earliness is not essential in the Spanish interior, but the number of ear rows shown by the new sweetcorn populations derived from Rastrojero needs to be increased if inbred lines are to be competitive.For the fresh corn market, ears should have at least 16 straight rows (Tracy, 2001).
Lazcano sh2 flowered later, had a higher kernel moisture and fewer ear rows than expected.In the sh2 background, the different performance of Lazcano compared to the other two field corn populations cannot be explained by mutant sweetcorn sh2 alleles since all three populations were crossed to similar sh2 donors.
This study shows that the differences between the new sweetcorn populations derived from Oroso, Lazcano or Rastrojero were stable, and differed from the original field corn populations.The new Lazcano-based populations performed slightly worse than the original Lazcano field corn, while the new Oroso-based populations showed improved performance over the original Oroso population.Revilla et al. (unpublished), studying different crosses between field corn and su1 populations, found that the differences among them were quite stable and barely affected by the genotypes from which they were derived.In other work involving crosses between field corn inbreds belonging to different genetic backgrounds, and su1 corn inbreds representing the variability of sweetcorn in temperate areas, Revilla et al. (2006) showed that the fitness of su1 depended on the sweet × field corn inbred combination.
With respect to the agronomic traits evaluated in the present work, the new sweetcorn populations derived from Rastrojero, Oroso and Lazcano did not show unfavourable values compared to the hybrid controls, except for ear rows (Table 4).The necessary breeding for table quality should be undertaken jointly with breeding for increasing the ear row number.
The data in Table 5 agreed with those of a previous study in which Oroso was identified as a suitable donor for improving the yield and yield components of su1 hybrids (Cartea et al., 1996a).Oroso can also supply favourable alleles for improving su1 se1 and sh2 varieties.Hopefully, Oroso su1 will show improved survival under cold conditions compared to su1 controls; Revilla et al. (1998) indicates that Oroso can improve the ability of some su1 inbreds to germinate and survive under cold conditions.Rastrojero might be able to supply a trait missing in these hybrids: resistance to ear damage by corn borers (Malvar et al., 2004).
In conclusion, the ability of field corn populations to improve sweetcorn was not greatly affected by the sweetcorn donor used.The field corn population chosen as the donor for improving sweetcorn must depend on the growing conditions in mind: Oroso appears to be more suitable for sweetcorn for northern Spain, while populations derived from Rastrojero seem to be more suitable for the Spanish interior.The potential of other field corn populations should be explored.Within each genotype and column, means followed by the same letter do not differ significantly (P > 0.05).

Table 1 .
Type of corn genotype and its origin

Table 2 .
Mean values of the mutant types across three populations derived from Oroso, Lazcano and Rastrojero, assessed over two years and at two locations

Table 3 .
Mean values for the three types of populations defined by their background field corn, assessed over two years and at two locations

Table 4 .
Mean values for the variables measured in the different corn varieties, assessed over two years and at two locations

Table 5 .
Mean yields of the corn varieties at Pontevedra and Zaragoza over the two years of the experiment